1. Field of the Invention
This invention relates to a constant velocity ratio universal joint of axially fixed (non-plunging) type, the joint comprising an outer joint member provided in its interior with a plurality of circumferentially spaced grooves extending as meridians about the axis of rotation of the outer joint member, an inner joint member disposed inside the outer joint member and provided on its exterior with grooves extending as meridians about the axis of rotation of the inner joint member, facing the grooves in the outer joint member in pairs, a plurality of balls disposed one in each facing pair of grooves in the joint members for torque transmission therebetween, and a cage of annular form disposed between the inner and outer joint members and having windows wherein the balls are received, to hold the balls with their centres in a plane, the centre lines of the grooves of at least some of the pairs thereof comprising at least two portions of different curvature as viewed in planes containing the axes of rotation of the joint members, with centres of curvature positioned symmetrically relative to each other on opposite sides of the ball centre plane when the joint is aligned.
Such fixed joints are commonly used at the outboard ends of the drive shafts of front wheel drive vehicles. Such joints have to be able to transmit high torques, and operate at large angles of articulation.
2. Description of Prior Art
GB 810 289 discloses a form of constant velocity ratio fixed universal joint wherein the grooves of each pair comprise arcuate centre lines whose centres of curvature are offset by equal amounts relative to the ball centre plane. A disadvantage of such joints is that towards the end of each groove the groove depth decreases, which limits the torque which can be transmitted when the joint is articulated to its maximum permitted angle. At maximum articulation, the large control angle (defined between the tangents to the balls and grooves at the points of contact therebetween) causes high axial forces to act on the cage. This may lead to overloading of the cage when the joint is subject to high loads at high articulation angles, e.g. when starting the vehicle on a hill or when trying to free a vehicle which has become stuck with the steering wheel on lock. Such overloading may possibly lead to jamming of the joint or even breakage thereof.
A further example of a fixed constant velocity ratio universal joint is disclosed in DE 37 00 868, wherein only half the pairs of groove are configured to provide a control angle as above referred to, by axially offsetting the centres of curvature of the arcuate centre lines of the grooves relative to the ball centre plane. The other groove pairs have the centre lines of the grooves concentric, which grooves, because of their uniform depth, are able to accommodate unchanged torque when the joint is articulated. From the point of view of production, providing two different types of groove is too expensive, particularly for mass produced joints for application in motor vehicles.
A further inadequacy of the joints above referred to is that the balls move radially relative to the cage through relatively long distances. The cage must therefore be relatively thick, as a result of Which the usable groove depth in the joint members is reduced and thus the torque which is able to be transmitted is further reduced. This can lead to fracture of the windows in the cage when fitting the balls.
A further type of joint of the kind first referred to is disclosed in DE 22 52 827, wherein the grooves have centre lines which comprise arcuate portions followed by tangentially extending straight lines. The centres of curvature of the arcuate portions of the grooves in each pair are offset by equal amounts on opposite sides of the ball centre plane when the joint is in the aligned (non-articulated) condition. The centres of curvature do not lie on the axes of rotation of the joint members. At maximum joint articulation, however, the above referred to disadvantages of reduced groove depth are even more significant. The large control angles which exist in the grooves whose centre lines lie in the plane of joint articulation when the joint is articulated to its maximum angle are not necessary for control of the joint, which control function is taken over by the grooves which cross one another in the plane perpendicular to the articulation plane. Axial forces acting on the cage disadvantageously generate increased friction and ball loads, with increased risk of joint fracture.